Currently parts of user list and group list command are actually
functioning as role listing, which is quite counter intuitive and
misleading.
This refactor change move role related logic to a single place of role
list command. It now allows role grants listing for user/group +
domain/project combinations.
If no user or group specified, it will list all roles in the system,
which is the default behaviour.
Change-Id: I4ced6df4b76f018d01000d28b4281ad9f252ffcc